Fuel efficiency of internal combustion engines can be substantially improved by varying the displacement of the engine. This allows for the full torque to be available when required, yet can significantly reduce pumping losses and improve thermal efficiency by using a smaller displacement when full torque is not required. The most common method today of implementing a variable displacement engine is to deactivate a group of cylinders substantially simultaneously. In this approach the intake and exhaust valves associated with the deactivated cylinders are kept closed and no fuel is injected when it is desired to skip a combustion event. For example, an 8 cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it is operating using only the remaining 4 cylinders. Commercially available variable displacement engines available today typically support only two or at most three displacements.
Another engine control approach that varies the effective displacement of an engine is referred to as “skip-fire” engine control. In general, skip-fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, a particular cylinder may be fired during one engine cycle and then may be skipped during the next engine cycle and then selectively skipped or fired during the next. In this manner, even finer control of the effective engine displacement is possible. For example, firing every third cylinder in a 4 cylinder engine would provide an effective displacement of ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders.
U.S. Pat. No. 8,131,445 (which is incorporated herein by reference) teaches a continuously variable displacement engine using a skip-fire operational approach, which allows any fraction of the cylinders to be fired on average using individual cylinder deactivation. In a continuously variable displacement mode operated in skip-fire, the amount of torque delivered generally depends heavily on the firing fraction, or fraction of combustion events that are not skipped. In other skip-fire approaches a particular firing pattern or firing fraction may be selected from a set of available firing patterns or fractions.
Vehicles require a method to disengage the engine from the drive wheels and vary the relative rotation rates between the engine and wheels. Various mechanisms can be employed to satisfy these requirements. In particular, vehicles often use an automatic transmission (to adjust the relative engine/wheel rotation rates) coupled to a torque converter (to disengage the engine and wheels). The torque converter uses a fluid coupling to transfer power from the engine to the remainder of the powertrain. The torque converter also typically includes a lockup or torque converter clutch (TCC) that provides a direct mechanical coupling in parallel with the fluid coupling. Engaging the TCC allows the torque converter to operate in a locked or partially locked state. This improves the vehicle fuel efficiency, since less power is lost in the torque converter. Generally the fluid and mechanical coupling work cooperatively using a single control input, the pressure of the hydraulic fluid within the torque converter, to transfer torque from the engine to the remainder of the powertrain. When the vehicle is operating in a steady-state cruising mode, such as open road highway driving, torque converter slip values in the range of 20-80 revolutions per minute (RPM) are typical. This slip results in a power and associated fuel economy loss, which can be estimated by taking the ratio of the torque converter slip to the engine speed. For example, if the engine is operating at 2000 rpm and the torque converter slip is 40 rpm, the efficiency loss in the torque converter is approximately 2% (40/2000).
The torque converter fluid can only transmit torque when the torque converter is slipping, since a mismatch between the rotation speeds of the torque converter input and output shafts is required for the fluid to transmit torque. In the case of a non-slipping, locked-up torque converter, the engine torque is transmitted by the torque converter clutch and a rigid mechanical connection exists between the engine and remainder of the powertrain. While this condition is advantageous from a fuel economy standpoint, since no power is lost in the torque converter, modern vehicle control generally seeks to avoid this condition because of its propensity to cause excessive noise, vibration, and harshness (NVH).
A potential problem with skip-fire engine control is that the non-uniform firing pattern results in increased noise, vibration, and harshness (NVH). In particular a vehicle powertrain is a naturally lightly-damped oscillatory system that can oscillate in response to rapid changes in the engine output torque, such as may be generated by a firing pattern of a skip-fire controlled engine. Various control systems have been proposed that operate an engine so as to avoid exciting the natural oscillatory frequencies of the powertrain or increase the slip in a torque converter so that these frequencies are not transmitted through the torque converter. In particular the slip of the torque converter may be increased to dampen any undesirable vibrations originating from a transition in the number of activated cylinders. Increasing slip has the undesirable effect of decreasing fuel efficiency, which negates some of the benefits of skip-fire control. It would be desirable to reduce torque converter slippage in a vehicle using a skip-fire controlled engine to further improve vehicle fuel efficiency.